delete more that one bookmark folder at a time
https://support.mozilla.org/en-US/questions/1255718 this shows how to delete more that one bookmark in the same folder at a time. Want to do same thing but from the folder directory tree on the left of the window.
Or make html bookmark file import overlay the current iternal firefox bookmarks so the previous exported bookmarks can be used and not be added as a additional copy at the bottom of the internal firefox bookmarks
Opaite Mbohovái (1)
In the left panel in the Bookmarks Manager (Library) you can only delete one folder at the time. If you want to delete more folders then you can select the parent folder in the left panel to access all its content in the right panel and to be able to select more folders.